refactor: improve branch cache logic

[RahulGautamSingh]

Context

• Closes: Improve branch cache logic #17663

Documentation (please check one with an [x])

• I have updated the documentation, or
• No documentation update is required

How I've tested my work (please tick one)

I have verified these changes via:

• Code inspection only, or
• Newly added/modified unit tests, or
• No unit tests but ran on a real repository, or
• Both unit tests + ran on a real repository

[RahulGautamSingh]

Current Flow:

1. Initialize branchCache = {}
2. if(repositoryCache === enabled && branchExisted)

• branchCache = findBranchCache(branchName)

if cache found assign it to branchCache
else create new branchCache with the fields branchName+sha, baseBranchName+sha

• syncBranchCache()
  -- processBranch() --

3. if new commit is made -> setBranchCommit()

Note:
setX fns logic flow:

1. try to find branchCache

if found update isX value
else return

not creating new branchCache here because if we wanted cache we would have done so before processing branch

[RahulGautamSingh]

I made a tiny modification:

• setBranchCommit and syncBranchCache have been converted to pure functions.
  Reason: This is good practise.

Will revert if you also think that it makes the code appear more complicated and less readable.

Aside from the two concerns:

1. using pure fns and
   2. Not creating branchCache and just returning when branchCache is not found in setX fns

I believe the implementation is in line with what you requested in the (#17663) 🤞.

I tested it on repos, and it works as intended.

[rarkins]

I think there's no harm in "writing" to the branch cache during a run even when cache is disabled as long as the cache file itself isn't written out at the end. We can simply think of it as branch state, which either gets written to cache if enabled or discarded.

[RahulGautamSingh]

New updates:
To get in line with the branch state terminology, I made some changes:

1. assign cached value to branch if it exists, (same as old)
2. if cache not found then create a minimal branchCache in the beginning only ie. before syncing cache

Minimal BranchCache:

{
branchName,
sha, 
baseBranchName, 
baseBranchSha
 }

After that we continue with : syncBranchCache -> processBranch -> if new commit --> setBranchCommit()

Note:
Now we have a minimal branchCache before processing branch in all these cases too:

1. branch cache not found
2. on first run ie. branchExisted=false
3. repositoryCache !== enabled

[RahulGautamSingh]

Dividing this PR into smaller PRs.